Revert "[compiler-rt] Don't reset non-default user handler if allow_user_segv_handler is true."

Summary:
On Android we still need to reset preinstalled handlers and allow use handlers later.

This reverts commit r304039.

Reviewers: eugenis

Subscribers: kubamracek, dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D34434

llvm-svn: 305871